Permeable membrane systems are known in the art and have been proposed for a variety of gas and liquid separations. In a typical membrane system, a feed stream is contacted with the surface of the membrane, and the more readily permeable component of the feed stream is recovered as a permeate stream and the less readily permeable component is recovered as a non-permeate, or retentate, stream.
Gas separations utilizing membranes are effected by contacting the feed stream with the surface of the membrane at an elevated pressure and withdrawing the permeate stream at a reduced pressure, relative to the elevated feed pressure. Significant factors in the design and overall efficiency of membrane systems are the total membrane surface area required for a given separation and the partial pressure difference across the membrane that is required to obtain a desired product quanity and quality. The design of membrane systems requires a balancing of these factors. That is, the greater the partial pressure difference, or driving force, across the membrane, the less is the membrane surface area required for a given separation. High pressure difference, low area operation necessitates the use of more expensive pumping equipment and higher pump operating costs, but enables membrane equipment costs to be kept relatively low. If, on the other hand, a lower driving force is employed, more membrane surface area is required, and the relative costs of the various aspects of the overall system and operation would change accordingly.
Membrane systems are often designed and optimized for full capacity, steady flow and composition conditions that are not always encountered in practice. When conditions exist that are different than the design conditions, the products recovered from the membrane system may contain undesirable concentrations of certain components. Under such conditions, different requirements exist with respect to partial pressure differences and membrane area in order to maintain a given product purity.
The problem of membrane control in turndown situations has been addressed in U.S. Pat. No. 4,806,132, issued to Campbell, which discloses a process for controlling permeable membrane separation systems when there are reduced product demand, or lower product purity requirements. The process of the above-identified patent operates by reducing the feed flow and the partial pressure driving force across the membrane. As a result, the product purity or flow is reduced. The patent further discloses that the driving force is reduced by reducing feed pressure or increasing permeate side pressure. The above-identified patent also discloses other known techniques for controlling product quality in turndown conditions including reducing the surface area of the membrane, i.e., by shutting down one or more membrane sections, or by employing a surge tank to handle variable demand requirements.
U.S. Pat. No. 4,397,661, issued to King, et al., discloses a process for the separation of fluids using at least two membrane permeator stages which can provide high turndown rates of permeate while maintaining substantially constant concentrations of at least one moiety in the permeate. In the process disclosed in the above-identified patent, a feedstream containing a permeating moiety and a slower permeating moiety is provided to each of a plurality of permeator stages containing selectively permeable membranes. Permeate from at least one of the stages is allowed to pass only when the combined permeate flow surpasses a predetermined rate, and such passage is terminated only when the combined flow drops below a predetermined rate.
U.S. Pat. No. 4,863,492, issued to Doshi, et al., discloses a gas permeation process and system for integrating a gas permeable membrane system with a multiple bed pressure swing adsorption system to produce a mixed gas product having a preset adjustably controlled gas ratio and a high purity second gas component. The patent discloses the use of a control valve on the permeate stream that can respond to changes in feed flowrate or composition in order to provide a constant throughput for the blended product. The feedstream is analyzed for flowrate and/or composition and the control valve is adjusted in response thereto.
In many instances, however, when the feedstream is subject to changes in feed composition, it is desirable to maintain the product purity of one of the effluent streams, i.e., permeate or non-permeate, since the product from the permeate membrane separation system may be used for further downstream processing. Moreover, it is often further desirable to maintain a relatively constant pressure as well as purity of the product stream despite fluctuations in feed composition. Feedstream compositional changes are also often accompanied by minor variations in feedstream flowrate, i.e., less than about .+-.20% of the design flow. In such cases, turndown in product flow is undesired. Accordingly, processes are sought for controlling the concentration of a component in an effluent stream from a permeate membrane gas separation system when the feed stream is subject to fluctuations in feed composition. Furthermore, processes are sought which can provide a product stream having relatively constant purity and pressure.